Firstly, You should ensure that you have checked your Facebook APP ID is correct as this will cause error messages if it is not setup correctly.
If you have checked your Facebook APP ID is correct: We have heard of a few issues with the Facebook comments API recently which has meant that occassionally Facebook comments do not show on OptimizePress based pages.
This is not an OptimizePress based issue - and is related to the Facebook API or comments system being temporarily down. Unfortunately this is outside of our control and hopefully the issue will be fixed as soon as possible.
You can always switch your pages to use WordPress comments if you are worried about the facebook comments reliability, or use both Facebook and WordPress comments so you have both sides covered.